This part is different to the one in later model bikes but it looks like it works on the same principle. It looks like the washer with the red arrow pointing to it has friction material on the back of it. There is one of these on the other side of the gear as well. Is the washer with the blue arrow a convex shape? If so, this is the one that applies the pressure that is supposed to prevent the gear from spinning. To increase the pressure I added another washer on top of the convex washer. There must be a small circlip that holds the assembly together. There is pressure on it so you'll need to use some sort of puller like I showed in
this thread to pull it apart and get it back together.